1-5
1-5. BLOOD VESSELS
The blood vessels are the closed system of tubes through which the blood flows. The arteries and arterioles are distributors. The capillaries are the vessels through which all exchange of fluid, oxygen, and carbon dioxide takes place between the blood and tissue cells. The capillaries are the smallest of these vessels but are of greatest importance functionally in the circulatory system. The venules and veins are collectors, carrying blood back to the heart.
a. The Arteries and Arterioles. The system of arteries and arterioles is like a tree, with the large trunk, the aorta, giving off branches that repeatedly divide and subdivide. Arterioles are very small arteries, about the diameter of a hair. By way of comparison, the aorta is more than one inch in diameter. An artery wall has a layer of elastic, muscular tissue that allows it to dilate and constrict. When an artery is cut, this wall does not collapse, and bright red blood escapes from the artery in spurts.
b. Capillaries. Microscopic in size, capillaries are so numerous that there is at least one or more near every living cell. A single layer of endothelial cells forms the walls of a capillary. Capillaries are the essential link between arterial and venous circulation. The vital exchange of substances between the capillary blood and the tissue cells takes place through the capillary wall. Blood starts its route back to the heart as it leaves the capillaries.
c. Veins. Veins have thin walls and valves. Formed from the inner vein lining, these valves prevent blood from flowing back toward the capillaries. Venules, the smallest veins, unite into veins of larger and larger size as the blood is collected for return to the heart. The superior vena cava, collecting blood from all regions above the diaphragm and the inferior vena cava, collecting blood from all regions below the diaphragm, return the venous blood to the right atrium of the heart. Superficial veins lie close to the surface of the body and can be seen through the skin.
